Retirement Plans and Other Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2020

Retirement Benefits [Abstract]

Schedule of net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged or amortized to the regulatory asset)

The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction or billed to electric plant participants) (dollars in thousands):

	Pension Benefits												Other Benefits
	Three Months Ended June 30,						Six Months Ended June 30,						Three Months Ended June 30,						Six Months Ended June 30,
	2020			2019			2020			2019			2020			2019			2020			2019
Service cost — benefits earned during the period	$	13,859		$	12,408		$	28,116		$	24,951		$	5,401		$	4,470		$	11,118		$	9,184
Non-service costs (credits):
Interest cost on benefit obligation	29,522			34,069			59,283			68,421			6,417			7,421			12,929			14,947
Expected return on plan assets	(46,915		)	(43,049		)	(93,721		)	(85,942		)	(10,019		)	(9,603		)	(20,038		)	(19,206		)
Amortization of:
Prior service credit	—			—			—			—			(9,394		)	(9,455		)	(18,788		)	(18,910		)
Net actuarial loss	8,295			10,053			17,306			21,292			—			—			—			—
Net periodic benefit cost (credit)	$	4,761		$	13,481		$	10,984		$	28,722		$	(7,595	)	$	(7,167	)	$	(14,779	)	$	(13,985	)
Portion of cost (credit) charged to expense	$	271		$	7,000		$	1,613		$	15,244		$	(5,056	)	$	(5,063	)	$	(10,512	)	$	(9,880	)